1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and particularly, to an LCD device capable of increasing an aperture ratio by way of micro-patterning upon forming an electrode line including a pixel electrode of the LCD device and reducing a process time taken by a micro-patterning process, and a fabrication method thereof.
2. Discussion of the Background Art
In general, a thin film transistor (TFT) is widely used as a switching device in semiconductor devices, display devices such as TFT LCD devices, and the like.
Among others, the TFT LCD device is recognized as the next generation high-tech display device with characteristics of low power consumption, high portability, technology-intensiveness and highly value-added aspect.
Among several types of LCD devices, demands of an active matrix type LCD device having TFTs as switching devices for adjusting power-on or power-off for each pixel are increased due to high resolution and capability of realizing videos.
In order to micro-pattern a pixel electrode electrically connected to a TFT, which is widely used as a switching device in a semiconductor device as well as in the LCD device, many technical problems such as lengthy process time, etching non-uniformity and the like, may occur. Especially, various difficulties often arise in achieving a high aperture ratio, which hinders the process of increasing brightness of the LCD device.
From this perspective, a related art LCD device will now be described with reference to FIG. 1.
FIG. 1 is a sectional view schematically showing an LCD device structure according to the related art. As shown in FIG. 1, an LCD device according to the related art includes a color filter substrate (not shown) with color filters, a TFT array substrate 11 facing the color filter substrate, a liquid crystal layer (not shown) interposed between the color filter substrate and the TFT array substrate 11.
Here, the TFT array substrate 11 includes thereon gate lines (not shown), gate electrodes 13a diverged from the gate lines, and a plurality of common electrodes 13b disposed in parallel to the gate lines with spaced gaps therebetween.
A gate insulation layer 15 is formed on the entire surface of the array substrate 11 including the gate electrode 13a. A semiconductor layer 21, which includes an active layer 17 and an ohmic contact layer 19 sequentially formed in an island shape, is formed on the gate insulation layer 15. Here, the active layer 17 is made of pure amorphous silicon (a-Si:H), and the ohmic contact layer 19 is made of impure amorphous silicon (n+a-Si).
On the ohmic contact layer 19, the LCD device further includes a data line 23, which crosses over the gate line (not shown) to define a pixel region, a source electrode 23a extending from the data line 23, and a drain electrode 23b spaced from the source electrode 23a. Here, the gate electrode 13a, the semiconductor layer 21, the source electrode 23a and the drain electrode 23b construct a thin film transistor (TFT) T.
In addition, a passivation layer 25 having a contact hole (not shown) for exposing part of the drain electrode 23b is formed on the entire surface of the source and drain electrodes 23a and 23b and the exposed portion of the active layer 17.
A pixel electrode 31a is formed on the passivation layer 25. The pixel electrode 31a is independently present in each pixel region, and contacts the drain electrode 23b via the contact hole (not shown). Here, the pixel electrode 31a is formed of indium tin oxide (ITO) as a transparent conductive material, and provided in plurality aligned in each unit pixel region and spaced apart from each other by a predetermined gap.
Accordingly, the plurality of common electrodes 13b and the plurality of pixel electrodes 31a formed on the TFT array substrate 11 are aligned horizontally with gaps therebetween, so as to make horizontal magnetic fields responsive to voltages applied thereto. Here, liquid crystal molecules located between the horizontal magnetic fields are affected so as to be driven by the magnetic fields.
Hereinafter, a method for fabricating the related art LCD device of FIG. 1 will be described with reference to FIGS. 2A to 2E.
FIGS. 2A to 2E are sectional views briefly showing sequential processes of a method for fabricating the LCD device of FIG. 1 according to the related art.
As shown in FIG. 2A, a gate line (not shown) and a gate electrode 13a perpendicularly extending from the gate line are formed on a transparent substrate 11. Here, a common line (not shown) disposed in parallel to the gate line is also formed on the substrate 11, in addition to the gate line and the gate electrode extending from the gate line. The substrate 11 also includes thereon a common electrode 13b extending from a common line, which is in parallel to the gate line and spaced therefrom by a predetermined gap.
Next, the gate insulation layer 15 is formed on the entire surface of the substrate 11 having the gate electrode 13a. The semiconductor layer 21, which includes the active layer 17 and the ohmic contact layer 19 sequentially formed in an island shape, is formed on the gate insulation layer 15. Here, the active layer 17 is made of pure amorphous silicon (a-Si:H), and the ohmic contact layer 19 is made of impure amorphous silicon (n+a-Si).
Afterwards, there are provided, on the ohmic contact layer 19, the data line 23 crossing over the gate line, a source electrode 23a extending from the data line 23, and a drain electrode 23b spaced apart from the source electrode 23a with a predetermined gap based upon the gate electrode 13a. Here, the gate electrode 13a, the semiconductor layer 21, the source electrode 23a and the drain electrode 23b construct a TFT T.
A passivation layer 25 made of an inorganic insulating material is formed on the entire surface of the substrate 11 having the data line 23, the source electrode 23a and the drain electrode 23b. 
Then, as shown in FIG. 2B, the passivation layer 25 is selectively etched out through a lithography process using photolithography and a patterning process, to form a contact hole 27 for exposing a portion of the drain electrode 23b. 
As shown in FIG. 2C, a transparent conductive material such as ITO is deposited on the passivation layer 25 having the contact hole 27, thereby forming a single-layer transparent conductive layer 31.
After coating a photosensitive material on the transparent conductive layer 31, an exposure mask (not shown), which defines a position where the pixel electrode is to be formed, is aligned on the photosensitive material layer (not shown). Lithography process and developing process for emitting infrared light to the photosensitive material layer through the exposure mask are executed so as to form a photosensitive layer pattern 33.
As shown in FIG. 2D, the transparent conductive layer 31 is selectively etched out through a wet etching process by using the photosensitive layer pattern 33 as a barrier layer, thereby forming the pixel electrode 31a. Here, although not shown, the pixel electrode 31a is provided in plurality so as to be aligned in each pixel region by being spaced apart with a predetermined gap. Also, the plurality of pixel electrodes 31a may alternate with the plurality of common electrodes 13b with predetermined spaced gaps therebetween.
As shown in FIG. 2E, after forming the pixel electrode 31a by selectively etching out the transparent conductive layer 31 through the wet etching process, the remaining photosensitive layer pattern 33 is removed completely so as to complete the fabrication of the TFT array substrate of the LCD device.
Afterwards, although not shown, the process of fabricating the LCD device is completed by executing a process of fabricating a color filter array substrate including a black matrix layer and a color filter layer and a process of forming a liquid crystal layer between the color filter array substrate and the TFT array substrate 11.
Considering the LCD device and the fabrication method thereof according to the related art, however, the following problems exist.
According to the LCD device and the fabrication method thereof according to the related art, the etching process used therein should be executed by considering etching capability according to the characteristic of a metal upon etching a single-layer metal layer, for example, ITO, molybdenum, titanium alloy or aluminum, which is used when forming the existing pixel electrode. Accordingly, the etching process becomes complicated. That is, etchant variation becomes drastic according to the type of metal involved, which makes it difficult to implement uniformity over limitations, and which results in a lower efficiency of the etching process and renders employment of a new metal difficult.
Also, for etching a metal layer having a single layer structure, etching uniformities at the upper and lower sides and right and left sides of the metal layer become harder to achieve due to defects, thereby making it difficult to realize micro lines in the etched product.
Further, when etching the single-layer metal layer, the metal layer may be damaged because it is already externally exposed. Consequently, formation of uniform lines becomes difficult, and a time taken for etching the metal layer is increased, thereby lowering productivity.
Thus, in order to form the pixel electrodes or other metal lines, such as the gate lines or the data lines, into micro electrodes each having a micro line-width w1, many technical problems such as an increase in the etching process time, difficulty in obtaining etching uniformity, metal damages and the like, may occur. For instance, various difficulties may arise in fabricating a display device needing high aperture ratios, which causes limitations on increasing the brightness of the display device.
Furthermore, the ITO as the transparent conductive material which is used in the related art LCD device is superior to transmittance but inferior to a contrast ratio, and hard to implement a line-width w1 below about 3.0 μm. If molybdenum titanium (MoTi) is used as a material for addressing such a problem, the contrast ratio may improve; however, a rainbow spot phenomenon where external light looks like a rainbow while it is reflected at a metal electrode and transmitted through a polarizer may occur. Consequently, to obviate the rainbow spot generated while such light is reflected at the metal electrode and transmitted through the polarizer, a low-reflective electrode, which can reduce reflectivity of an electrode, is urgently required.